(1) Field of the Invention
The present invention relates to a process for melting an extremely low carbon steel in an oxygen-blowing converter. More specifically, the invention relates to a process for speedily and appropriately melting an extremely low carbon steel containing not more than 100 ppm of carbon through a converter blowing without relying upon a vacuum degassing refining.
(2) Description of the Prior Art
The significance in the production of an extremely low carbon steel with not more than 100 ppm of carbon by only a converter is mentioned below with referring to a steel sheet to be extremely deeply drawn which is frequently used as a steel product for the automobiles.
When the extremely low carbon cold rolled steel sheet for extremely deep drawing is subjected to heat treatment in a continuously annealing equipment, it has been conventionally necessary to adopt a complicated process in which subsequent to heating and quenching, reheating is performed for effecting overaging treatment. However, a sufficient non-aging property is not necessarily assured stably through the overaging treatment.
As regards this point, it has been recently made clear that when an extremely low carbon steel sheet having the content of carbon of, for instance, around 30 ppm is used, sufficient deep drawability and stable non-aging property can be obtained through simply heating and cooling treatment only.
As obvious from the above example, while various excellent characteristics can be obtained depending upon kinds of steels by lowering the content of carbon down to not more than 100 ppm, such an extremely low carbon steel can be first melted by subjecting a molten steel containing not more than 500 ppm of carbon which has ordinarily been refined in a steelmaking furnace such as a converter to a vacuum refining treatment of RH or DH system or the like for a long time of period.
However, when the time period for the vacuum treatment is prolonged, it becomes difficult to assure the temperature of a molten steel which is fit to the succeeding continuous casting step due to the drop in temperature during the treatment. If the converter steel tapping temperature before the vacuum treatment is largely elevated to cope with this problem, the durable life of the refractory material of the converter is shortened, thereby resulting in problems such as increased cost and efficiency drop due to repair.
On the other hand, the top- and bottom-blown converter has been developed to improve the metallurgical characteristics of the top-blown converter, that is, to reduce the oxidation of iron and manganese in a molten steel, and it is its fundamental requirement to mainly increase the force of stirring the molten iron with a bottom-blown gas.
The reason why T.Fe in the slag are reduced in the top- and bottom-blown converter as compared with the top-blown converter and the reason why the oxidation rate of Mn in the molten iron decreases are that the reaction in the molten iron or between the molten iron and the molten slag approaches an equilibrium state due to the increased molten iron-stirring force so that preferential decarburization effectively proceeds.
However, it has been heretofore considered to be economically impractical to effect decarburization down to not less than 100 ppm in the carbon content in the oxygen top-blown converter. This is because when O.sub.2 gas is blown into the molten steel having a reduced carbon content from a top-blowing lance, oxidation reaction of the molten iron takes place together with the decarburization reaction so that with the decrease in the carbon content the oxidation reaction of the iron exceeds the decarburization reaction to increase the loss of iron through oxidation and lower the yield of an iron source to a large extent.
Therefore, the limit for the decarburization in the oxygen top-blown converter is generally considered to be 150-250 ppm.
In order that the preferential decarburization required in melting the extremely low carbon steel may advantageously and continuously proceed, the reduction in the CO partial pressure as well as the approaching to the equilibrium through increased stirring force must be done. For proceeding the decarburization reaction under the control of the oxidation reaction, there has been proposed another approach in which an inert gas such as nitrogen or Ar gas is mixed into O.sub.2 gas to be fed from the top-blowing lance, that is, the O.sub.2 gas is diluted with the inert gas to lower the CO partial pressure. This approach is that the partial pressure of carbon monoxide in the furnace which is considered to be ordinarily at an atmospheric pressure is lowered through dilution to cause the following reaction: EQU C+1/2O.sub.2 .fwdarw.CO (g)
preferentially to the oxidation reaction of iron: EQU Fe+1/2O.sub.2 .fwdarw.FeO
thereby attaining the decarburization.
However, according to this process, the attainable limit value is around 120 ppm at the largest, and an extremely low carbon steel containing not more than 100 ppm of carbon cannot be obtained. Further, this method is inevitably accompanied with the loss of iron through oxidation, and consequently the yield of iron is lowered to 90% or less.
Moreover, according to this process, a mixed gas of oxygen and an inert gas is blown only at a flow rate of 1-1.5 Nm.sup.3 /min per 1 ton of the molten iron by using a horizontal-blowing tuyere or a bottom-blowing tuyere. When the mixed gas is blown at such a flow rate, that of O.sub.2 is naturally less than 1-1.5 Nm.sup.3 /min so that the decarburizing rate is low and such a process is clearly unsuitable for the rapid refining.
In addition, there has been reported a process for melting an extremely low carbon steel containing about 150 ppm of carbon in an oxygen bottom-blown converter such as a Q-BOP. However, since a coolant, lime and so on are bottom-blown besides oxygen in the bottom-blown converter, the installation cost and the refining cost become higher due to the blowing equipment and an attendant equipment for blowing powder without being clogging.